ASK TOM WHY.

Dear Tom, At what temperature does salt become...

November 08, 2001|By Tom Skilling, chief meteorologist at WGN-TV.

Dear Tom,

At what temperature does salt become ineffective in melting ice and snow on the roads?

Nick Ulm, Monticello, Ind.

Dear Nick,

Salt has proven itself as the most effective cost-efficient melting material for icy or snow-covered roads. However, as the temperature falls below 10-15(degrees), salt loses its melting power and becomes ineffective. If the salt is treated with calcium chloride, an exothermic (heat releasing) reaction occurs, and snow and ice will melt at considerably lower temperatures.

Chemicals exist that melt snow and ice at temperatures well below zero, but they are very expensive and would quickly deplete a city's snow-removal budget. Since most of Chicago's snow occurs with temperatures in the 20s or lower 30s, untreated salt does an effective job. At lower temperatures, adding calcium chloride to the salt helps continue the melting process.

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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.